Enemy NPC (mob)

Something like random Youkai appearing at night as like another mob, probably less HP than a zombie and does 1 1/2 hearts to non-armor.

Would be beneficial as a new enemy to experience since the only thing that would probably kill you in the wild would be mobs in general swarming you, arrow spam, or being trapped.

Oooh, maybe difference level of youkai in different places? Maybe more dangerous ones in one biome than another?

The problem with custom mobs is that it would require a mod for players to install as well as installing on the server, otherwise players would be unable to join the server.